In the art of relatively small gasoline engines, it has been commonplace to mount the fuel tank higher than the carburetor so that there can be a gravity feed of the fuel to the carburetor. There are some types, however, where it is desirable to place the fuel tank lower than the carburetor, and in this instance a pump is needed to convey the fuel. For example, in the recreational water vehicle known as the Jet Ski (a registered trademark owned by the present assignee) which has a shallow bottom, the fuel tank should be installed at a low level to avoid a high center of gravity and the resulting instability, and the carburetor of the engine is generally at a point higher than the fuel level in the tank.
As another example, in large-tired tricycles (or buggies) which are required to have the capability to run in swamp or bog areas and the like, it is necessary to install the carburetor at a high level, which also results, in many cases, in the fuel level being lower than the carburetor.
Fuel supply systems for the above vehicles cannot, of course, be of a gravity supply type, and a suction type fuel supply system in which a fuel pump is provided between the fuel tank and the carburetor is normally used. FIG. 1 illustrates a typical prior art suction type fuel supply system. In FIG. 1, the curburetor 2 of the engine 1 is installed at a higher level than the fuel tank 3, and the feed of fuel to said carburetor is effected by the suction of a fuel pump 4. Connected in the fuel line between the fuel tank 3 and the fuel pump 4 are a change-over valve 5 and a filter 6, and the valve 5 and the fuel tank 3 are connected to each other by two supply lines 7A and 7B which have suction ends at different levels in the tank 3. When the fuel level in the fuel tank 3 is at or above the higher level A, the valve 5 is operated to connect the supply line 7A to the pump 4, and when the fuel level drops below the level A, the valve 5 is operated to connect to the supply line 7B (reserve line) having the lower suction end. The level B is the level of the suction end of the supply line 7B (or the minimum level). Fuel between the level A and the level B is used as the reserve fuel.
The supply line from the fuel pump 4 to the carburetor 2 is branched, one branch leading to the carburetor 2 and the branch 8 being connected to an upper level in the fuel tank 3 and constituting a return circuit.
In the prior art system of FIG. 1 described above, if the engine were turned off for an extended period (e.g., for several days or a week), the fuel initially filling the supply line up to the carburetor may evaporate or drain out. This would cause air or fuel vapor to fill the space in the fuel supply line, and the resulting condition of the system would be such that, without priming, it would be difficult to pump up the fuel in order to restart the engine, which would, in turn, result in a longer startup time and increased battery load. In particular, if the engine pulses such as negative pressure fluctuations in the crankcase were used to operate the fuel pump 4, the startup problems would be more pronounced due to small discharge of the pump during the startup when the engine turns over slowly.
It is a primary objective of the present invention to avoid the aforementioned problems encountered with the conventional fuel supply systems and to provide a fuel supply system which can easily and quickly supply fuel to the carburetor even if the engine is started with air or vapor in the supply system, to achieve easy startup.